There are many types of supplemental steering controls known in the art. Many of the known supplemental steering controls use a pressure storage device such as an accumulator to store pressurized fluid that can be used to provide pressurized fluid for steering in the event the main steering control fails. In other steering systems, a steering control valve having a HMU (hand metering unit) is used during emergency conditions. In these systems, the HMU is used to provide pressurized fluid to steer the machine in the event, the main steering pump flow is lost.
In other steering control systems, a differential steer mechanism is used to provide the steering of the machine. In many of these differential steering units, individual steer motors are connected to the differential steer mechanism and operate to either increase or decrease the speed of the respective outputs that provides the driving force to the machine. By increasing or decreasing the speed of one of the outputs relative to the other output, the machine turns in the direction of the slowest output speed. In differential steered machines, each of the outputs are always under a continuous driving force, even when the machine is being steered. By using two different steering motors connected to the respective outputs, during an emergency steering control it is possible to provide stored pressurized fluid to one or the other of the steering motors to steer the machine. Since fluid motors normally require a volume of pressurized fluid to provide the force on a continuous basis to provide the needed differential between the two outputs, it is many times not practical to try to store the needed pressurized fluid.
In other differential steer machines, a single fluid motor is connected to the differential steer mechanism. In these differential steer mechanisms, the single steer motor provides an input into the differential steer mechanism to increase the speed of one of the outputs and simultaneously decrease the speed of the other of the outputs. When the single steer motor is not receiving pressurized fluid to cause rotation of its output, the drive outputs from the differential steer mechanism are mechanically locked together by the non-rotating fluid motor. As noted above with respect to the differential steer mechanisms having two steering motors, it is not practical to use accumulators for storage of pressurized fluid for emergency steering since the volume of pressurized fluid needed for a longer duration of emergency steering is too large.
It has also been known to provide a complete redundant emergency steering control but these systems are normally cost prohibitive. In machines not having differential steering mechanisms, it is well known to use brakes on the drive outputs to individually slow one output relative to the other in order to steer the machine. In these machines, when the one drive output is braked, the power is transferred across a standard differential to the other drive output.
In machines having differential steer mechanisms with only one steering motor, attempting to brake one output is not successful since both outputs are under a continuous driving force. Consequently, the machine will not respond to attempts to steer by braking since the non-rotating fluid motor has the two outputs locked together. In these differential steered machines, even attempts to install larger brakes of very high capacity will not provide adequate steering by braking. Even if it did provide some steer control, the size of the brakes would be cost prohibitive.
The present invention is directed to overcoming one or more of the problems as set forth above.